In the pursuit of effective early diagnosis and functional cure for osteoarthritis (OA), we are confronted by the lack of understanding of how certain molecular activities in cartilage tissue are linked to the deterioration of macroscopic function. This project explores the nanostructure and nanomechanics of cartilage extracellular matrix (ECM), an intermediary between the two. The overall objective of this study is to uncover how decorin and biglycan, two potential critical matrix molecules, contribute to cartilage ECM hierarchical structure and depth-dependent mechanics, as well as their influences in the initiation of OA. This study contributes to our long term goal of building a comprehensive structural and mechanical knowledge that bridges between cartilage molecular activities and tissue-level mechanical function, which is necessary for developing early diagnosis and functional cure/repair that directly target the molecular-level phenomena of OA. To accomplish this goal, we will use an array of nanostructural and atomic force microscopy-based nanomechanical assays to examine knee cartilage from decorin-deleted and biglycan-deleted transgenic mice, against the normal control. These assays include nanoindentation to extract cartilage modulus and relaxation time constant, our customized nanorheometer to measure hydraulic permeability, and scanning electron microscopy imaging to quantify collagen fibril diameter. In combination with conventional biochemical and radiology analyses, we will obtain necessary structure-mechanics knowledge on these mice. Through comparing outcomes from these mouse models, we will test our central hypothesis that decorin and biglycan govern the age and depth-dependent mechanical properties of cartilage through regulating the collagen network structure, and thus, affect cartilage mechanical instability that pre-dates OA. This study encompasses two specific aims that focus on the roles of decorin and biglycan in two age stages. In Aim #1, we focus on the skeletal development period of mice from 1 to 12 weeks age to identify the roles of decorin and biglycan in cartilage structure/mechanics during maturation. In Aim #2, we focus on potential OA symptoms in mice 2 - 12 weeks after the destabilization of medial meniscus surgery at 12 weeks age. We aim to determine whether decorin-null mice are susceptible to surgery-induced OA and develop a nanomechanics-based evaluation method for the detection of early OA. Successful completion of this study will provide the mechanics-based knowledge regarding the molecular pathogenesis of OA involving decorin and biglycan. Methods developed here will offer a novel infrastructure for further investigating the nanomechanics-related symptoms of OA in other synovial tissues of murine models or human knees.